The present invention relates to a synchronous machine, in particular a generator for a motor vehicle.
More particularly, it relates to a synchronous machine which has an excitation system composed of a plurality of electrically excited individual poles in a rotor, for example in form of claw poles excited by at least one common excitation coil, in which for compensation of the magnetic stray flux, in the free spaces between the claw poles oriented axially toward one another, permanent magnets are introduced on pole plates mounted on the axial ends of the rotor, which are supported against centrifugal forces and held by a holder.
Synchronous machines of this type are known in the art. One of such machines is disclosed for example in the German patent document DE 89 05 353 U1. In this machine the permanent magnets located in the free spaces between the oppositely located claw poles are oriented by holding plates against the centrifugal forces and magnetized so that they counteract the stray flux. The holding plates are inserted at the peripheral side in widening of the claw poles and fixed, while they are supported at the side facing the central axis of the machine on a non magnetic sleeve which surrounds the excitation stage. This construction of the permanent magnets requires not only substantial expenses for the parts which depend on the number of poles on the synchronous machines, but also requires a substantial mounting expense for the holding plates. The rotor of the synchronous machine is therefore expensive to manufacture. Moreover, the permanent magnets are not clearly oriented in the axial direction against the axial blow forces.
As shown in U.S. Pat. No. 5,543,676 it is known to insert the U-shaped permanent magnet between the excitation coil and the claw pole, so that the side legs of the permanent magnets are introduced in the free spaces between the claw poles. These permanent magnets extend only to the axial center of the free spaces, since a permanent magnet is inserted from each tip of a claw pole.
Also, a meandering and ring-shaped holder for the permanent magnets is known, which is prefabricated as a unit and during the mounting of the rotor on the pole core is fitted with the excitation winding, before the pole plates are mounted from both axial ends with their claw poles. The claw poles are introduced in the axially open receptacles of the holder as shown in the patent document UK 2 281 665 A. This known holder has a complicated construction and is expensive to manufacture. Moreover, the permanent magnets and the holder itself are not clearly secured against the centrifugal forces and axial blow forces.